The present invention relates to a keyphone system, and specifically to a method of displaying duration of a call that is made over a central office line (hereinafter, referred to as "C.O. line").
In conventional keyphone systems, there exists a problem that a user is not able to know exactly how long he has been talking over the C.O. line and since the C.O. line does not provide a signal that specifies an exact time at which the other party on the other end of the C.O. line started to respond to his call, even though there are some keyphones with special functions capable of calling duration of a call made over a C.O. line, call-duration is displayed based on a system-defined time from which counting of the call-duration is started, so that an exact duration of the call that used the C.O. line cannot be known to its user.